Method and system for variable cam timing device

Abstract

Methods and systems are described for an engine with a cam torque actuated variable cam timing phaser. Phaser positioning control is improved by reducing inaccuracies resulting from inadvertent spool valve and/or phaser movement when the spool valve is commanded between regions. In addition, improved spool valve mapping is used to render phaser commands more consistent and robust.

The invention claimed is: 
     
       1. A method, comprising:
 during selected conditions,
 ramping a spool valve coupled to a cam torque actuated variable cam timing phaser from a detent region to a retard region via adjusting a duty cycle applied to a solenoid; 
 mapping a transitional region between the detent and retard regions based on phaser movement away from a locked position, the phaser movement responsive to the ramping; and 
 storing the duty cycle to memory in response to detecting retarding motion of the cam torque actuated variable cam timing phaser. 
 
 
     
     
       2. The method of  claim 1 , wherein the selected conditions include one of a green engine condition, a threshold distance elapsed since a last mapping, a threshold duration elapsed since a last mapping, a DFSO condition, and phaser position error being higher than a threshold. 
     
     
       3. The method of  claim 2 , wherein ramping the spool valve includes varying the duty cycle applied to the solenoid of the spool valve at a constant positive rate. 
     
     
       4. The method of  claim 3 , wherein varying the duty cycle includes starting at a 0% duty cycle. 
     
     
       5. The method of  claim 4 , wherein the mapping includes ending the duty cycle applied to the spool valve solenoid in response to the phaser movement away from the locked position. 
     
     
       6. The method of  claim 1 ,
 further comprising moving the cam torque actuated variable cam timing phaser to a locking position in response to degradation of a detent circuit. 
 
     
     
       7. The method of  claim 1 ,
 further comprising indicating degradation of a detent circuit in response to cam torque oscillations being greater than a threshold. 
 
     
     
       8. The method of  claim 7 , further comprising comparing cam torsional torque pulses to values stored in controller memory. 
     
     
       9. The method of  claim 8 , further comprising determining an offset trim based on the cam torsional torque pulses. 
     
     
       10. The method of  claim 1 , further comprising moving the spool valve through a retard region between cam torsional pulses in response to a cam locking command when engine speed is less than a threshold. 
     
     
       11. The method of  claim 10 , further comprising adjusting the duty cycle to zero in response to the cam locking command when engine speed is greater than the threshold. 
     
     
       12. The method of  claim 11 ,
 further comprising adjusting the cam torque actuated variable cam timing phaser to a position advanced of a locking position in response to the cam locking command when engine speed is greater than the threshold. 
 
     
     
       13. A method, comprising:
 providing closed loop cam position control and adjusting a cam timing phaser to a target holding position; and
 in response to a request for locking the cam timing phaser for more than a threshold duration, 
 operating in a learning mode to map a transitional region between a detent region and a retard region of a spool valve coupled to the cam timing phaser, the transitional region mapped based on cam timing phaser motion out of a locked position relative to spool valve motion through the transitional region. 
 
 
     
     
       14. The method of  claim 13 , wherein the request for locking the cam timing phaser for more than the threshold duration includes not requesting a cam phaser advance command for more than the threshold duration. 
     
     
       15. The method of  claim 13 ,
 further comprising mapping the transitional region in response to engine shutdown conditions being present. 
 
     
     
       16. The method of  claim 13 ,
 further comprising adjusting the cam timing phaser to a lock position in response to engine shutdown conditions being present. 
 
     
     
       17. The method of  claim 13 , further comprising, adjusting a phasing command applied after operating in the learning mode based on a learned updated border. 
     
     
       18. The method of  claim 17 , wherein adjusting the phasing command includes limiting a maximum retard command based on the learned updated border. 
     
     
       19. An engine system, comprising:
 an engine cylinder including valves; 
 cams coupled to a camshaft for actuating the valves; 
 a variable cam timing phaser for adjusting valve timing, the phaser actuated using torque from the cams; 
 a solenoid driven spool valve for adjusting a position of the phaser; and 
 a controller with computer readable instructions stored on non-transitory memory for:
 receiving a command for moving the phaser out of a locked position to a desired unlocked position; 
 measuring cam torsional pulses; 
 estimating an error between an actual unlocked position of the phaser relative to the desired unlocked position; and 
 in response to the error being higher than a threshold,
 operating in a learning mode with the phaser commanded to the locked position to update a map of a transitional region between a detent region and a retard region of the spool valve based on motion out of the locked position relative to spool valve motion through the transitional region. 
 
 
 
     
     
       20. The system of  claim 19 , wherein the controller includes further instructions for:
 indicating degradation of a detent circuit in response to the cam torsional pulses.